Airport scanners are one of the main tools in deterring and preventing contraband, such as explosives, guns and knives, and other prohibited items, from being brought on board an airplane. Current scanners include line scanners where bags are transported between a detector array and one or more X-ray sources. These scanners can miss the detection of contraband if the contraband is of a certain shape, and the bag containing the contraband is oriented in a certain direction. Such scanners rely heavily on the operator's alertness to possible threats. CT scanners are much better at scanning bags because they can produce views through a bag in many different directions, and can incorporate automatic threat detection. Additionally, dual energy CT scanners have been developed to reduce the false alarm rate associated with the automatic threat detection by providing atomic number measurements of scanned objects in addition to density measurements. Different dual energy implementation schemes have been proposed or developed. One dual energy implementation scheme is to use a switching high voltage power supply. See for example, Bernard M. Gordon, et al., “Dual energy power supply,” U.S. Pat. No. 5,661,774, assigned to the present assignee. Recently one CT manufacturer has developed a dual-source CT scanner, which uses two x-ray tubes placed within one single gantry and is capable of dual energy imaging. See, for example, T. G. Flohr, et al., “First performance evaluation of a dual-source CT (DSCT) system,” European Radiology 16 (2), 256-268 (2006). The widely used passive “sandwich” detectors, e.g. the detectors used in the system described in H. Takeo, et al., “A new FCR image processing function: energy subtraction FCR9501 ES/FCR DX-A,” Fuji Comput. Radiogr. Tech. Rev. 4, 1-24, 1995., provide dual energy imaging capability with a single X-ray source. All these implementations require special designs and are usually very costly. In addition, these implementation schemes are not suitable for easily upgrading existing single energy CT scanners, particularly of interest to security applications, to obtain dual energy imaging capability for reducing the false alarms from the automatic threat detection system. Efforts are constantly directed towards finding ways to decrease the costs of CT security scanners while maintaining many of the advantages of dual energy CT scanning systems, and to develop a simple and easy way to convert current single energy scanners into dual energy scanners.